Commercial versions of these can be quite expensive so here is a modified microwave transformer for demagnetising tools, there are more than one way do do this but the basic idea is to break the circuit of the laminated core and use the primary windings only.
The top of the core can be removed completely along with the secondary windings to create what is known as an E core and inverted to use the bottom face as the usable surface.
I have chosen to remove the secondary windings and make a saw cut though the top, I could have cut though the side but did not think of at the time, the cut is then filled with a non conducting material to prevent the ingress of anything that could bridge the gap, I did this because the mounting feet made it easier this way.
Mine has a working surface of 90 x 70 and work is passed along the direction of the laminations to demag, it is important to make a slow pass and continue until about a foot away before switching of.
A simple plywood box was made to the height of the core with a switch and warning light (the latter is not really necessary as it makes a rather loud buzzing noise when switched on).
A thin top cover is used to prevent marking of finished work and would ideally be made of fibre but not having any I used sturdy cardboard (those beer boxes again) which has been varnished.
I am sure this would overheat if left on but most things can be dealt with in under a minute.
This can demag quite large items and if I have a HSS tool that needs treatment I just draw the whole tool holder over it.
I have made a mini version for smaller items and this is often taken out to jobs outside the shop for drill bits etc.